The present invention relates to a spraying system, and more particularly, to a paint spraying system using a robot having a movable arm.
Paint spraying operations performed on workpieces moving along a conveyor, such as those used in automotive painting plants, have historically been performed manually. As a workpiece, such as an automobile body, moves along the conveyor, a worker carrying a spray gun would move the gun back and forth across the body while walking a short distance along the conveyor. Sufficient hose would be provided to enable the worker to freely the move spray gun and, in order to avoid the excess hose from striking the body being painted, the worker would grasp a portion of the hose with a free hand and hold it behind his or her back while spraying.
In a typical painting plant, the paint color being sprayed at any given paint station may be changed as much as once for every workpiece passing by on the conveyor. In past painting systems, in order to select the color to be sprayed, a rack was provided having a plurality of hoses, each one supplying a different paint color, and the worker could take a hose and connect it to the spray gun by means of a quick disconnect fitting. The worker would then spray a small amount of the new color of paint through the gun prior to spraying the workpiece in order to clear out any of the old paint remaining in the gun.
In an improved painting system, such as the one shown in U.S. Pat. No. 3,674,205, the worker was provided with a control panel by which a particular color could be selected by means of pneumatically or solenoid controlled valves which would supply the desired paint color from one of a plurality of paint supply hoses to a single inlet hose to the spray gun. In this system, in order to change the paint color, the inlet hose had to be purged with a solvent and refilled with a new paint color prior to spraying the workpiece such that the new color of paint was not contaminated with residual paint remaining in the hose and gun, which could lead to off-color workpieces.
In recent years, the spraying operation in painting plants has been automated through the use of robots. The robot typically has a spray gun mounted to the end of a pivoted arm with the spray gun and arm articulated for various degrees of movement. A central hose used to supply paint to the spray gun must be held in close contact to the arm in order to prevent the hose from whipping into the workpiece being painted as the spray gun is moved back and forth.
When it is desired to change the color being sprayed, a valve similar to that used for manual systems operates to switch between colors, however, as with the manual systems, before the new color can be sprayed, the central hose leading to the spray gun must be purged with a solvent to eliminate the old color in order to ensure that a mixed color is not sprayed. This clean-out operation is estimated to cost in excess of $140,000 per robot in solvent and lost paint per year.
In addition, in order to ensure that the paint being sprayed is a consistent color, a recirculatory line is provided from each of the individual paint supply lines for each color to keep the paint flowing within the supply line when the valve leading to the spray gun is closed. If the control valves should malfunction such that one color gets mixed within the recirculatory line of another color, the plant must be closed and the lines purged of any remaining paint in order to ensure that the proper colors will be supplied by the particular lines. This type of accident can be costly both in down time of the plant and in lost paint.
One solution to eliminating the waste of solvent and paint resulting from the problems described above is to provide a rack having a plurality of hoses such that the robot can select a given hose in order to spray a selected color. The Japanese published disclosure 60-36365 discloses such a solution for a painting system which uses a robot. In the Japanese system the robot selects an individual hose which is connected close to the sprayer and then uses the sprayer with the hose dangling freely downwardly. This system suffers from the problem of the long unrestrained hose being free to whip around as a robot arm moves the sprayer, such that the hose may contact the workpiece being sprayed. This problem is particularly severe if the robot is moved rapidly back and forth across the workpiece, as is typical in automated automotive painting operations.
Thus, there is a need for a paint spraying system using a robot in which the amount of paint lost during color changing operations is minimized as well as minimizing the possibility of accidental color mixing. In addition, there is a need for a paint spraying system in which the movement of a paint supply hose is properly controlled during movement of the robot in order to avoid any contact between the paint supply hose and the workpiece being painted.